This invention relates to a high refractive index, high dispersion optical glass having a refractive index (nd) of 1.78 or over, an Abbe number (νd) of 30 or below, and a partial dispersion ratio (θg, F) of 0.620 or below. This invention relates also to optical elements such as lenses and prisms obtained by using this optical glass.
There is a large demand for a high refractive index, high dispersion optical glass as a material for optical elements such as lenses of various types. As optical glasses having a refractive index (nd) of 1.78 or over and an Abbe number (νd) of 30 or below, known in the art are glass compositions disclosed by Japanese Patent Application Laid-open Publication Nos. Sho 52-25812 and 2004-161598, and WO2004/110942.
An optical system using such optical glass is mounted on an optical product such as a digital camera. For improving chromatic aberration in such optical glass, it is desired for the optical glass of a high refractive, high dispersion region to have a relatively small partial dispersion ratio.
For this reason, from the standpoint of utility in optical design, there has been a strong demand for an optical glass having high refractive index, high dispersion characteristics and a small partial dispersion ratio.
Particularly, a high refractive index, high dispersion optical glass having a refractive index (nd) of 1.78 or over and an Abbe number (νd) of 30 or below is strongly desired for.
The above mentioned publications satisfy the above described refractive index and Abbe number. The optical glasses disclosed specifically in these publications, however, do not satisfy conditions that the glass comprises SiO2 and Nb2O5 as essential components, wherein an amount of Nb2O5 in mass % is more than 40%, and further comprises, in mass % on oxide basis, less than 2% of K2O and one or more oxides selected from the group consisting of B2O3, TiO2, ZrO2, WO3, ZnO, SrO, Li2O and Na2O wherein a total amount of SiO2, B2O3, TiO2, ZrO2, Nb2O5, WO3, ZnO, SrO, Li2O and Na2O is more than 90% and TiO2/(ZrO2+Nb2O5 is less than 0.32.
In an optical system for a digital camera, a spherical lens is generally used. A spherical lens is produced by heating and forming a lens preform material by using a mold having a shape which is closely similar to the shape of the lens, and polishing the obtained lens preform material. In a high refractive index, high dispersion optical glass, devitrification tends to occur during forming of the lens preform material by heating (reheat press molding) and, therefore, an optical glass which has a high resistance to devitrification is desired for.
On the other hand, for correcting spherical surface aberration, an aspherical lens is useful. As a method for producing an aspherical lens cheaply, precision press molding is known. The precision press molding is a method according to which glass for a lens preform material is heated and thereby softened and a high precision mold surface is transferred to the glass by pressing the mold. Since the mold is exposed to a high temperature environment, the forming surface of the mold tends to be oxidated and eroded or a release film provided on the surface of the forming surface of the mold tends to be damaged and the high precision forming surface of the mold thereby cannot be maintained or the mold itself tends to be damaged. In such a case, the mold must be exchanged with the result that frequency of exchange of the mold increases and a large scale production of the lens preform material at a low cost becomes difficult. For this reason, from the standpoint of preventing such damage to the mold, maintaining a high precision forming surface of the mold for a long period of time and enabling precision press molding at a low pressing force, it is desired for an optical glass for a lens preform material to have as low a glass transition temperature (Tg) as possible.